The executive monitor functions as a supervisor of all aspects of the signal transmissions of an MLS to insure that the MLS integrity performance requirements are being met. The executive monitor must be sensitive to relatively small variations in a wide range of signal parameters. It must have sufficient accuracy and cognition to eliminate false alarms, while retaining the ability to respond to catastrophic failures in less than one second. It should be capable of verifying its own ability to measure out of tolerance conditions and must be sufficiently reliable to satisfy the continuity of service requirements of the MLS. In order to guarantee that the probability of the transmission of erroneous information by the MLS is less than 0.5.times.10.sup.-9, the executive monitor should operate completely independently of any control cooperation or other shared relationship with the MLS being monitored.
Prior monitoring methods or systems lack the complete physical and electrical isolation of the monitor from the ground electronics unit that is needed to avoid hidden dependencies between the monitor and the ground unit. The majority of monitoring methods in use employ a plurality of distributed internal sensors to measure signal status at critical points in the signal formation process of the ground unit. The measurements of the sensors are compared with prescribed standards and the findings of the comparisons are consolidated to imply a global measure of the MLS signal integrity.
Certain of the prior methods use an R.F. receiver to derive the signal parameter being monitored from the R.F. signal transmitted by the MLS. For example, U.S. Pat. No. 4,764,772, issued Aug. 16, 1988, for "Scanning Beam Microwave Landing System", discloses a monitoring method employing a monitoring facility having some of the attributes of an airborne MLS receiver. The monitor facility includes an R.F. unit, an I.F. unit, a data demodulator and means for determining the time of arrival of the scanning beam. The R.F. unit is a single channel device tuned to the frequency of ground unit being monitored. synchronizing signals for the monitor are derived from the ground unit. No provisions are made for monitoring MLS signal parameters other than data transmitted by the scanning beam antenna and the arrival time of the scanning beam.
Such prior monitoring methods and means are clearly not statistically independent of the MLS ground unit. Also, prior monitor systems are incapable of validating their own performance, so that it is necessary to employ redundant monitors to verify monitor integrity.
It is an object of the present invention to provide a method and means for monitoring all aspects of the signals transmitted by an MLS ground unit, which method and means are statistically independent of the MLS ground unit.
It is another object of the invention to provide a method and means for monitoring the signal transmissions of an MLS ground unit, which method and means includes the ability to self verify the integrity of the monitoring process being performed.
Other objects and advantages of the invention will become evident as a complete understanding thereof is gained from the following complete description and accompanying drawings.